A new species of copepod belonging to the genus Idiodiaptomus Kiefer, 1936, is described from northeastern Argentina. All that was known about this genus was based on the brief, original description of Idiodiaptomus gracilipes (Van Douwe, 1911). This was the only known species of this genus created by Kiefer (1936) to accommodate this bizarre copepod. The new species, I. dussarti n. sp., closely resembles I. gracilipes, but differs from it mainly in the shape, segmentation, and armature of the prosome and urosome, as well as in several details of the fifth pair of legs, both in the male and in the female. The distinguishing characters of the new species include, in the male, the head-like tip of the exopodal distal process of the left fifth leg and the large rounded process on the second exopodal segment of the right fifth leg. Females can be easily recognized by their 2-segmented urosome and the presence of a sub-tetrahedrical ‘hump’ on the fourth pedigerous somite.